1. Field of the Invention
The present invention relates to high frequency power amplifiers and more particularly to techniques for combining, monolithically or otherwise, individual power amplifiers to achieve power combining and impedance transformation.
2. Description of the Related Art
The design of high frequency power amplifiers with reasonable power levels, efficiency and gain remains one of the major challenges in the pursuit of a single-chip integrated transceiver. Although several advances have been made in this direction, the design of a truly integrated power amplifier on a lossy substrate, such as silicon or silicon germanium has been an elusive goal.
Multiple external components such as bonding wires and external baluns have been used as tuned elements to produce output power levels in excess of 1 W using CMOS transistors. See e.g., K. C. Tsai and P. R. Gray, “A 1.9 GHz, 1-W CMOS Class-E Power Amplifier for Wireless Communications,” IEEE Journal of Solid-State Circuits, vol. 34, no. 7, pp. 962–969, July 1999[1]; and C. Yoo and Q. Huang, “A Common-Gate Switched, 0.9 W Class-E Power Amplifier with 41% PAE in 0.25 μm CMOS,” Symposium on VLSI Circuits Digest, pp. 56–57, Honolulu, June 2000[2]. Similar performance levels have been achieved with Si-Bipolar transistors. See, e.g. W. Simbürger, et al, “A Monolithic Transformer Coupled 5-W Silicon Power Amplifier with 59% PAE at 0.9 GHz,” IEEE Journal of Solid-State Circuits, vol. 34, no. 12, pp. 1881–1892, December 1999[3]; and W. Simbürger, et al., “A Monolithic 2.5V, 1 W Silicon Bipolar Power Amplifier with 55% PAE at 1.9 GHz,” IEEE MIT-S Digest, vol. 2, pp. 853–856, Boston, June, 2000[4].
Moreover, alternative technologies for active devices with higher breakdown voltages and higher substrate resistivity have been used to increase the output power and efficiency of integrated amplifiers. For example, LDMOS transistors with a breakdown voltage of 20V have been used on a semi-insulating substrate, but still this design delivers only 200 mW. See, Y. Tan, et al., “A 900-MHz Fully Integrated SOI Power Amplifier for Single-Chip Wireless Transceiver Applications,” IEEE Solid-State Circ., vol. 35, no. 10, pp. 1481–1485, October 2000[5]. Further, GaAs MESFET's on insulating substrates have been used to integrate power amplifiers. J. Portilla, H. Garcia, and E. Artal, “High Power-Added Efficiency MMIC Amplifier for 2.4 GHz Wireless Communications,” IEEE Journal of Solid State Circuits, vol. 34, no. 1, pp. 120–123, January 1999[6]. Unfortunately, these technologies are significantly more costly and more difficult to manufacture than conventional silicon-based transistor technologies, such as CMOS.
A summary of these prior achievements in the design of high-frequency, low voltage power amplifiers is provided in Table 1:
TABLE 1Freq.PoutSupplyPAEWirebondExternalActiveRef.(GHz)(W)Voltage%Inductor?Components?DeviceNo.1.91.0241YESYESCMOS[1]0.91.01.941YESYESCMOS[2]0.95.04.559NOYESSi Bipolar[3]1.91.42.555NOYESSi Bipolar[4]0.90.2549NONOSOI LDMOS[5]2.40.25779NONOMESFET[6]
Two significant problems in the design of a fully-integrated high speed solid state power amplifier using conventional silicon technologies such as CMOS are (1) the low resistivity of the lossy substrate which increases the loss of on-chip inductors and transformers; and (2) the low breakdown voltages of the transistors. These. problems are exacerbated as the minimum feature sizes of the transistors (such as CMOS) are scaled down for faster operation.
More particularly, the high conductivity of lossy substrates causes long metal lines, including conventional spiral inductors fabricated on the same substrate, to be very lossy in terms of power. If the metal lines are made wide to reduce resistance, the capacitive coupling effect between the metal and substrate will drain part of the current to the substrate, thereby increasing power dissipation. On the other hand, if the metal lines are made narrow enough to effectively overcome this problem, the metal resistance will significantly increase, again absorbing (dissipating) a significant portion of the power.
The low breakdown voltage of conventional transistors such as CMOS, for example, limits the maximum allowable drain voltage swing of the transistor. This makes it necessary to perform some form of impedance transformation to achieve a larger output power. For example, a ±2V drain voltage swing delivers only 40 mW to a 50Ω load if no such impedance transformation is performed. While impedance transformation can be achieved using a 1:n transformer, unfortunately, an on-chip spiral 1:n transformer on a standard CMOS substrate is very lossy and will degrade the performance of the amplifier greatly. Alternatively, an on-chip resonant match could be used, but this technique also results in significant power loss.
In sum, as all high frequency power amplifiers ostensibly require some inductors—essentially long metal lines—for matching purposes, connections for the supplies, and some form of power combining, conventional power amplifier circuits tend to be very power inefficient and not commercially viable above certain power and frequency levels.
Thus, it would be highly desirable to have a low cost, fully integratable topology for a power amplifier that can be fabricated with low cost, silicon-based processes and that can provide significant output power levels in the microwave and millimeter-wave frequency ranges. It would also be desirable if such a topology could be implemented with discrete power amplifiers as well as monolithic integration techniques. Ideally, this architecture would also be useful in the design of both lossy substrate IC's as well as non-lossy substrate IC's.